'It’s extremely common for defendants to be investigated for additional crimes.'

John and Carolyn Jackson leave U.S. District Court in Newark following a court hearing in May 2013.John O'Boyle/The Star-Ledger

NEWARK — A federal prosecutor said today that a grand jury is investigating the May 2008 death of an adopted son of an Army major and his wife to decide whether the child was a homicide victim.

Major John E. Jackson, stationed at Picatinny Arsenal, and his wife Carolyn are already facing child endangerment and assault charges for abusing the boy and two other adopted children for years by force-feeding them hot sauce and red pepper flakes and denying them water.

During a two-hour hearing in U.S. District Court, Assistant U.S. Attorney Melissa Jampol told Judge Katharine Hayden the government could bring a new indictment if a grand jury finds evidence of the more serious charge of homicide.

“The focus of the grand jury investigation is an ongoing investigation into the death of” the boy, Lampol said.

The age of the boy, identified in court papers as “J.J. 2,” has not been disclosed by prosecutors because he is a minor.

Herbert Waldman, the attorney for Carolyn Jackson, accused prosecutors of abusing the grand jury process to bolster the child-endangerment case against the Jacksons.

“It’s extremely common for defendants to be investigated for additional crimes,” she told the judge.

Hayden is expected to rule on the defense challenge Wednesday.

The disclosure of the second grand jury investigation came as Hayden agreed to a defense request to prevent prosecutors from telling jurors the boy died when the trial starts in September.

Hayden said any mention of the boy’s death would confuse and mislead a jury and would be “unfairly prejudicial” to the Jacksons. “The government may not introduce evidence that J.J. 2 died,” she said.

Prosecutors argued that the Jacksons “contributed” to the boy’s death and that the death was a crucial piece in a pattern of abuse that spanned nearly five years, from August 2005 to April 2010.

The couple was arrested in April 2013 and their children were placed in the custody of state child welfare officials. Their parental rights to one of their biological children and the remaining two adopted children – ages 8 and 6 – have been terminated, prosecutors said, with liberal visitation of the two biological children.

The family lived at the Picatinny Arsenal in Morris County from 2005 to 2010.

The indictment claims the couple enlisted their biological children in the “neglectful and cruel” abuse of the adopted children.

Two of the children, including “J.J. 2,” suffered broken bones and were denied prompt medical attention, the indictment says.

One biological child was ordered to keep an eye on one of the foster children to prevent the child from sneaking a drink of water out of sinks and toilet bowls, the indictment adds.

The Jacksons’ biological children – now 16, 14 and 12 – were told that they were “training” the adopted children to behave through physical assaults and cruel discipline, the indictment added.

Medical professionals, police and child welfare officials in Oklahoma, New Jersey and Indiana were misled about the source of the children’s injuries, prosecutors say.

In court papers, prosecutors say the Jacksons’ alleged abuse was discovered by an emergency room doctor who treated “J.J.2” and another adopted child and “recognized that the two biologically-unrelated children’s severe malnutrition and other health issues suggested child abuse.”